Extensible chainwheel for a bicycle powertrain assembly

ABSTRACT

A driving sprocket, or chainwheel, for a bicycle crankset having a toothed ring whose ring is divided into a plurality of segments, the segments being borne by a frame that is rotationally movable about a crankset axle, each sprocket segment being movable between a low position and a high position, a system for transferring each segment to transfer said segment from one position to another, and an indexing mechanism to control the system for shifting the segments. Each segment of the driving sprocket is floating mounted with respect to the frame and is guided in its transfer movement along a trajectory having a radial component and a tangent component.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an extensible driving sprocket for a bicycletransmission assembly.

2. Description of Background and Relevant Information

Extensible driving sprockets, or chainwheels, in bicycle transmissionassemblies are known to perform a progressive gear shift.

For example, French Patent Publication FR 961 243 describes a drivesprocket having four pinions for driving the chain that are radiallyextensible.

For this type of construction, the chain follows a polygonal trajectoryin the area of the drive sprocket, with substantial variations indistance from the axle of the sprocket which result in a correspondingsubstantial variation in the pedaling force.

French Patent Publications FR 1 140 134 and FR 1 542 594 describeanother constructional embodiment, according to which the sprocket doesnot have pinions but instead radially extensible ring segments.

The drawback to these types of construction is controlling thetransitory phases. It is readily understood that at the non-extensiblestate the segments are contiguous and that in their various states ofextension, they leave between them a space that is equal to a wholenumber of the pitch of the chain. However, in the case of a radialextension, there is the drawback of controlling the space between twocontiguous segments during the transitory phases, i.e., during thepassage of the segments to the following phase of extension orretraction.

Another flaw in these known constructions is that they seek to multiplythe number of transmission ratios, i.e., the number of positions forextending the segments. However, the further apart the sprocket segmentsare, the more difficult it is to change the segment positions, and themore perceptible is the previously mentioned phenomenon of polygonaltrajectory.

Another flaw is that the mechanism for radially extending the segmentsis located toward the middle of the segment. It directly sustains theforces resulting from the tension of the chain. Furthermore, it isoverhangingly stressed each time a segment is partially engaged with thechain. Added to this is the fact that the sprocket applies pressure tothe taut strand of the chain, i.e., the strand that has the maximumstress.

SUMMARY OF THE INVENTION

An object of the invention is to propose an extensible drive sprocket,or chainwheel, that improves the gear shift.

Another object of the invention is to propose an extensible sprocket inwhich the segments work under better conditions.

To this end, the invention proposes a driving sprocket for a bicyclecrankset having a toothed ring in which one pitch of the teeth are incorrespondence with one pitch of the chain, the ring being divided intoa plurality of segments, the segments being borne by a frame that isrotationally movable about a crankset axle, each sprocket segment beingmovable with respect to the frame in a plane perpendicular to thecrankset axle between a low position and a high position, in which it isspaced further from the crankset axle, thus defining a small diameterstate and an imaginary large diameter state of the sprocket, a systemfor transferring each segment to transfer said segment from one positionto another, and an indexing mechanism to control the system for shiftingthe segments. Each segment of the sprocket is floatingly mounted withrespect to the frame and is guided in its transfer movement along atrajectory including a radial component and a tangent component.

The transfer system and the indexing mechanism ensure a change of stateof the sprocket between a small diameter state in which all of thesegments are in the low position and a large diameter state in which allof the segments are in the high position. The passage from one state tothe other occurs by successively transferring the segments from oneposition to the other at the moment of rotation of the sprocket when thesegment is not engaged with the chain, i.e., during its raising phase.

The invention is based on the following observation. By displacing thesegments one by one along a trajectory that is both radial andtangential when they are being raised or lowered, it is possible toremain substantially synchronized with the chain pitch for each changeof state of the sprocket. The transfer of the chain from one state tothe other of the sprocket occurs by maintaining the teeth of thesegments in correspondence with the chain pitch, with an acceptableclearance, particularly when a segment having a low or high positiondifferent from the segment preceding it comes under the taut strand ofthe chain. The change of state of the sprocket therefore occurssmoothly, with no hooking or forcing of the chain.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with reference to the followingdescription and the attached drawings related thereto, and in which:

FIG. 1 is an overall view of a bicycle and schematically shows a drivingsprocket according to the invention in the retracted state;

FIG. 2 shows the bicycle with its driving sprocket in the extendedstate;

FIG. 3 shows the frame of a first constructional embodiment of thesprocket;

FIG. 4 is a cross-sectional view of the frame;

FIG. 5 is a perspective view of a plate seen from the outside;

FIG. 6 shows in perspective the plate seen from the inside;

FIG. 7 shows a sprocket segment;

FIG. 8 is a side view of a control lever;

FIG. 9 shows the lever seen from the front;

FIG. 10 shows the return spring of a segment;

FIG. 11 shows in perspective the device for guiding the levers mountedon the frame;

FIGS. 11–14 show the guiding device in its various positions;

FIG. 15 shows the sprocket with all of the segments in the low position;

FIGS. 16 and 17 show the two raising phases of a segment;

FIG. 18 is a view of the sprocket with all of its segments in a highposition;

FIG. 19 schematically shows the electronic system for controlling theservomotor;

FIG. 20 corresponds to another constructional embodiment of thesprocket;

FIGS. 21 and 22 show another constructional embodiment of the sprocket;

FIGS. 23 and 24 relate to another constructional embodiment.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a bicycle 1 that has, as is conventional, a frame 2, frontand rear wheels 3 and 4, as well as a transmission system 5. Thetransmission system is composed of a driving chain 6 that is driven inrotation by a crankset 8 that is movable about a crankset axle 9 borneby the frame. The chain in turn drives the rear wheel by means of acassette of sprockets 10 having varying numbers of teeth. A rearderailleur schematically shown as 11 guides the transfer of the chainfrom one sprocket to another.

The crankset 8 has, as known, a sprocket, i.e., a driving sprocket orchainwheel 13, the periphery of which has teeth in correspondence withthe pitch of the chain 6. The terms driving sprocket, drive sprocket,and chainwheel are used interchangeably herein for the particularembodiment being described.

In order to change the expansion ratio, the chainwheel 13 is constructedwith movable segments, the position of which is modified with respect tothe crankset axle. The segments can occupy two stable positions thatcorrespond to various diameter states of the chainwheel.

In the embodiment shown in the drawings, the chainwheel 13 has a toothedring that is formed of four segments 14 a, 14 b, 14 c, 14 d, eachcovering a ninety degree angle.

FIG. 1 shows the four segments of the ring in their low position. Inthis position, the segments are adjacent and form a continuous toothedring. This corresponds to the small diameter state of the ring.

In FIG. 2, the four segments 14 a, 14 b, 14 c, 14 d are in their highposition that corresponds to the large diameter state of the ring.

As known, in the position of large diameter, the segments are separatedby a distance corresponding to a whole number of pitch in order to keepthe chain continuously meshed on the teeth of the chainwheel.

For example, if one considers a chainwheel having 44 teeth in its smalldiameter state, formed of four segments of 11 teeth each, it is possiblethrough the extension of the segments to create a distance of one chainpitch between two consecutive chainwheels. This results in an imaginarychainwheel of 48 teeth, i.e., a chainwheel having an expansion of 48teeth. If the segments are displaced along a strictly radial trajectory,there is discontinuity in the pitch of the chainwheel teeth during thetransition between two segments having different positions. Thisdistance corresponds approximately to one chain half-pitch. In theseconditions, the chain rises temporarily on the teeth until the end ofthis transitory period in which it regains a correct meshing position.

The invention, on the contrary, provides displacing the segments one byone along a trajectory that has a radial component and a tangentialcomponent.

Thus, with reference to the previous example, during the change of stateof the chainwheel from a state of 44 teeth to an imaginary state of 48teeth, the segments are displaced one by one along a radial componentthat aligns them along the radius of the imaginary chainwheel of 48teeth, and a tangent component. The tangent component is determined suchthat at the junction of the two adjacent segments, one in the positionof 44 teeth, the other in the position of 48 teeth, the teeth of the twosegments are in correspondence with the teeth of the chain. In practice,good results were obtained by displacing the segment along a tangent oralmost tangent trajectory that displaces the segment in question in therotational direction of the chainwheel by an additional fraction thatcompensates for the spacing of the teeth at the junction of the twochainwheel segments induced by the difference in radius. In other words,care is taken not to introduce any pitch or fraction of an intermediarychain pitch between the two segments.

Following the rotation of the chainwheel, the other segments changeposition along the same radial and tangent trajectories that space themone from the other by a space corresponding to one chain pitch. In thelarge diameter state of the chainwheel, the segments of 11 teeth havebetween them a space of one chain pitch, giving a total of 48 imaginaryteeth.

By proceeding in this manner, it was noted that inversely, during thepassage of the chainwheel from its imaginary large diameter state to itssmall diameter state, the trajectory of the segment described in theopposite direction maintained a space corresponding to one chain pitchbetween two consecutive segments, one in the position of 48 teeth andthe other in the position of 44 teeth, with an acceptable clearance.Since the other segments successively follow the same trajectory, thespaces between two consecutive segments disappear until the chainwheelcompletely returns to its state of 44 teeth.

Thus, under these conditions, the correspondence between the chainwheelteeth and the chain pitch is constantly maintained, particularly duringall of the transitory phases of the segment extension or retraction,with an acceptable clearance. The change of state of the chainwheeloccurs therefore under very good conditions, even though it occurs onthe taut strand of the chain. As a matter of fact, there is nochainwheel change per se. Furthermore, the chain encounters no noteddiscontinuity in the teeth, particularly during a change of state of thechainwheel.

In the high position of the chainwheel, the segment teeth do not followa circular curvature precisely. Nevertheless, for the example mentionedhereinabove, the variation in radius is on the order of 2%, which ispractically imperceptible in terms of the pedaling speed.

Advantageously, the change of position of a segment is caused in theraising phase of the segment in question under the taut strand of thechain. Under these conditions, the segment can be led along severalpossible trajectories, as long as it has reached its final high or lowposition when it comes under the taut strand of the chain.

Advantageously, the change of state of the chainwheel induces arelatively small variation in the linear speed of the chain, andtherefore of the expansion. Generally, this variation is on the order of9% in the present example. Preferably, the driving sprocket, orchainwheel, according to the invention is associated with a cassette ofsprockets or gears whose variation in the number of teeth between twosuccessive gears is on the order of 18% for the variation ratio in thearea of the driving sprocket to remain less than the variation ratio inthe area of two successive gears.

As an example, good results were obtained for a road bicycle having sucha driving sprocket associated with a cassette of eight gears having 11,13, 15, 18, 21, 25, 30 and 36 teeth, respectively. Advantageously, therear derailleur is built according to the European Patent Publication EP558 425, and the gear shifts at the front and rear are guided by anelectronic control system ensuring a synchronized control of the gearshifts in the area of the driving sprocket and of the rear derailleur.

This system is provided to function in the following manner. Starting,for example, with the highest transmission ratio, the first gear shiftis a change of state of the driving sprocket from its large diameterstate to its small diameter state, the second is a simultaneous changeof gears and of a change of state of the driving sprocket to return itto its initial state, then again a change of state of the drivingsprocket alone, and so on.

The following table shows the transmission ratios and the variations ofratios system adapted to a road bicycle.

State of the Number of Variation of driving sprocket gear teethTransmission ratio the ratio % 48 11 4.36 44 11 4.00 9.09 48 13 3.698.33 44 13 3.38 9.09 48 15 3.20 5.77 44 15 2.93 9.09 48 18 2.67 10.00 4418 2.44 9.09 48 21 2.29 6.94 44 21 2.10 9.09 48 25 1.92 9.13 44 25 1.769.09 48 30 1.60 10.00 44 30 1.47 9.09 48 36 1.33 10.00 44 36 1.22 9.09

The range of transmission ratios varies from 4.36 up to 1.22, with anoverall ratio of 3.57.

If this table is compared to a conventional system, one can note thatthere is no longer any overlap in the succession of the various ratios.Furthermore, the variation of the ratios is much more even and centeredon a value of 8%, which improves of the comfort and the flexibility inusing the system. Additionally, the range of ratios is wider.

Furthermore, one can note that a range of transmission ratios equivalentto the range of a conventional system could be covered with only sixgears by eliminating the larger diameter gears.

By reducing the number of gears with respect to a conventional system,one decreases the drawbacks related to an incorrect alignment of thechain. Indeed, this alignment flaw occurs over a more reduced angularamplitude.

One also reduces the bulkiness of the gear cassette on the hub body,which allows spacing apart the flanges for fastening the spokes on thehub, thereby resulting in a better balancing of the spoke tensions and agreater rigidity of the wheel. There is also a reduction in weight inthe area of the gears.

Compared to a conventional system, it is certain that there are heremore ratio changes in the area of the secondary sub-assembly.

However, these changes of ratios occur alone or in combination with asingle gear change. Furthermore, they induce a more reduced ratiovariation than in the conventional case. Finally, they are caused by anexpansion of the driving sprocket, or chainwheel, and not by aderailleur acting on the taut strand of the chain. The output istherefore much better and the transition times are shorter.

This embodiment is not limiting. Specifically, other values for thenumber of teeth could be used.

According to the constructional embodiment of the driving sprocket, orchainwheel, that will now be described, the displacement of a segmentfrom one position to the other occurs with a double shifting movement.This movement allows controlling the trajectory of the segment along thetwo radial and tangent components. The embodiment that will be describedrelates, in a non-limiting manner, to a driving sprocket/chainwheel of44 teeth formed of 4 segments having 11 extensible teeth for animaginary state of 48 teeth.

In order to simplify the description, the elements will be designated byone reference numeral and, if necessary, by a reference character a, b,c or d related to a segment. From one segment to the other, the sameelements will be designated by the same reference numeral but adifferent reference character.

The chainwheel has a central support frame 16 that bears the foursegments. The frame is shown in FIG. 3. Depending on the figure, theframe 16 is a flattened structure having an approximately square form.Along the sides of this square form, the frame has zones 17 a, 17 b, 17c, 17 d that are provided to allow the guiding of the ends of twoadjacent segments of the chainwheel.

Given the normal rotational direction of the frame, the zone 17 a has,forwardly and rearwardly, guiding surfaces 18 a and 19 a for each of theforward and rearward zones of a segment. The rearward surface 19 aextends to the edge of a cavity 20 a provided for a return spring thatis described below. At the edge of the cavity, a bore 21 a is providedfor the fastening of the previously mentioned spring. Three other bores,22 a, 23 a, 24 a, respectively, are adapted to be used as a housing fora guiding pin, the journal axle of a shifting lever and a guidingabutment, respectively. These various elements are describedhereinafter.

The other zones are similar to what has just been described.

Between the cavities 20 a, 20 b, 20 c, 20 d, the frame 16 has lighteningopenings 25 a, 25 b, 25 c, 25 d. These openings are optional, their formis not limiting.

Toward the crankset axle, a groove 26 a is machined approximatelyhalfway between the bore 23 a provided for the shifting lever and thecrankset axle. This groove is centered on the bore 23 a. Its function isto limit the angular movement of the lever which is described below.

Plates, such as the plate 29 a, are provided to be assembled oppositethe guiding surfaces of two adjacent segments, particularly 19 a and 18b, so as to leave a guiding space for the forward and rearward zones oftwo adjacent segments. These plates have bores 30 a, 31 a thatcorrespond to the bores 24 a and 22 b. The plates are assembled to theframe 16 by any appropriate means, for example, screws and spacers. Onthe back, the plates have a guiding abutment 45 a for the rearward zoneof the segment traversed by the bore 30 a.

To assist in understanding the drawings, the plates are not shown in thefollowing figures, only the guiding abutment is seen.

A crank arm 32 is affixedly connected to the frame 16 which, in thecenter, has a square bore 33 for the crankset axle and the other crankarm. Another constructional embodiment of the cranks can also be used.

The segment 14 a will now be described with reference to FIG. 7, theother segments are identical. The segment 14 a generally has the form ofa crescent with a forward zone 34 a, a median zone 35 a and a rearwardzone 36 a, given its normal rotational direction marked by the arrow R.

At its periphery, the segment 14 a has a toothing 38 a with 11 teethmade in correspondence with the chain pitch. In the present case of foursegments, the teeth cover a quarter of the teeth of the chainwheel. Theteeth extend along a circular curvature centered on the crankset axlewhen the segment is in a low position, i.e., the position of FIG. 1. Asan alternative, the curvature could be centered on the crankset axlewhen the segment is in a high position or yet in an intermediateposition.

Preferably, the last tooth 39 a of the segment is machined along itsdescending surface and the depth of the space between the teeth whichfollows it is increased. The object of this arrangement is to allow thechain to dip along a direction tangent to the segment that is rearwardof the segment. This facilitates the transfer of the chain during aretraction of the segments.

In the forward zone, the segment 14 a has a guiding slot 40 a in theshape of a bean. The slot is oriented approximately along a radialdirection, with the central portion of the bean oriented toward therearward zone.

A guiding pin 41 a, housed in the bore 22 a of the frame, extendsthrough the slot 40 a, as shown in FIGS. 15–18. The pin 41 a circulatesin the slot 40 a without a transverse clearance, except for theoperational clearance, and, with its ends, the slot limits thedisplacement of the forward zone 34 a of the segment during its changeof position. The radial length of the slot is approximately equal to theamplitude of radial displacement of the segment, with the exception ofthe pin diameter. In the low position of the segment, the pin guides theslot so that the first tooth 37 a of the segment is spaced by one chainpitch from the last tooth of the preceding segment that is also in thelow position. In this manner, the chainwheel has 44 teeth. In the highposition, the pin guides the slot so that the first tooth 37 a is spacedfrom the axis of the hub by a distance corresponding substantially tothe radius of a chainwheel of 48 teeth, and so that it is spaced fromthe tooth that precedes it by a distance equal to one chain pitch, thepreceding segment being in the low position.

A recess 43 a is located at the end of its rearward zone 36 a of thesegment. The edge of the recess takes support on the guiding abutment 45a that is located on the back of the plate, such as plate 29 a, e.g.(see FIG. 5), when the rearward zone 36 a is in the low position.

At the base of the segment, i.e., towards the crankset axle, the segmentis demarcated by a ramp 48 a provided to cooperate with a shifting leverthat is described below. The ramp 48 a has a general direction that isperpendicular to a radial direction. It has two slopes 49 a, 50 a thatare each located toward a forward and rearward zone of the segment andform therebetween a dihedral angle whose apex is slightly inferior to aflat angle.

At its rearward end, the slope 50 a is stopped by a return 51 a adaptedto be used as an abutment for the shifting lever. When the segment ispositioned on the frame 16, the return 51 a and the bore 23 a for thepivoting axis of the lever are approximately aligned in the same radialdirection.

In this rearward zone of the segment, a bore 52 a is provided for areturn spring that is described below. The bore 52 a is located on thesegment in its low position, between the slot 40 a and the recess 43 a,so that a traction exerted by the spring can have a return action in thedirection of the crankset axle on both of the forward and rearwardzones.

A shifting lever 53 a for changing the position of the segment 14 a isshown in FIGS. 8 and 9. The lever has two arms 54 a and 55 a extendingon both sides of a pivoting axle 56 a and forming therebetween an angleslightly greater than a right angle. The pivoting axle 56 a is housed inthe bore 23 a of the frame 16. The longest arm 54 a has a head 58 aprovided to cooperate with the ramp 48 a of the segment. The length ofthe arm 54 a is determined as a function of the ramp 48 a to bring theteeth of the rearward zone 36 a of the segment approximately along theradius of a sprocket of 48 teeth.

At an equal distance from the pivoting axle 56 a, each of the arms 54 aand 55 a bears a stop 60 a, 61 a for indexing the lever. The stop 60 a,located on the same side of the axle 56 a as the head 58 a, is called anextension stop. The other stop 61 a that is closer to the crankset axleis called a retraction stop.

A rod 63 a, located on the shortest arm, opposite the stop 61 a, movesin the groove 26 a of the frame. In one of the stable positions of thelever, the rod is at the bottom of the groove 26 a.

Each of the segments is elastically returned to the low position andmaintained in this position by a return spring that connects it to theframe 16. For the segment 14 a, a spring 66 a is provided with its twoends 67 a and 68 a housed in the bores 52 a of the rearward zone of thesegment and 21 a of the frame, respectively. As for the body of thespring, it is housed in the cavity 20 a of the frame. According to thepreferred constructional embodiment of the spring shown in the figures,the body of the spring has two coils 69 a, 70 a, connected together andat the ends by arms 71 a, 72 a, 73 a. Under these conditions, the spring66 a is non-linear and has a small space requirement. Its return forceincreases rapidly over an amplitude of extension that corresponds to itsinitial pre-stressing. Also, its return force is relatively stable.

An indexing mechanism is provided to control the shifting of the leversand therefore the transfer of the segments from one position to another.This mechanism is affixedly mounted, for example, to the frame 2 of thebicycle. It has a plate 75 that is mounted, for example, to the bearingsurface of the crankset axle, and which is tightened with the tighteningnut of the axle. The plate carries a movable indexing connecting rod 76that is articulated about an axle 77 borne by the plate. Seen from theside, the axle 77 is in the vicinity of the trajectory of the pivotingaxles of the levers and, in particular, of the axle 56 a of the lever 53a. The connecting rod has a base 78 opposite the axle 77. The base 78has an end support surface that is substantially perpendicular to thelongitudinal direction defined by the connecting rod and, preferably, ithas a flexible zone 79 that is oriented toward the exterior of thechainwheel. According to the embodiment shown, the connecting rod isdriven by a rod 80 that is itself attached to the output cam 81 of aservomotor 82 that is electrically actuated. Other means could also beused, such as a rotatable or linear electromagnet, or a piezoelectricactuator.

The servomotor is guided by an electronic control system that isdescribed below. Depending on the angular position of the connecting rod76, the base 78 is provided to oppose the indexing stops of the leveror, on the contrary, to avoid the stops in question during the rotationof the driving sprocket/chainwheel.

FIGS. 12 and 13 show the connecting rod in the position where it opposesthe extension and retraction stops, respectively, i.e., the stops 61 aand 60 a for the lever 53 a.

FIG. 14 shows the connecting rod in an intermediate position where it isout of the reach of the various stops.

The transfer of the segment 14 a from one position to the other is shownin FIGS. 15–18.

As shown in FIG. 15, the segment 14 a and the other segments areinitially in the low position and are maintained in this position bytheir respective return springs. In this position, the teeth of thesegments are contiguous. The chainwheel generally functions like a smalldiameter chainwheel.

The levers, particularly the lever 53 a, are in the stable positionshown in FIG. 15. Under the effect of the spring 66 a that returns thesegment 14 a to the low position, the head 58 a of the arm 54 a isreturned toward the crankset axle, and the end 61 a of the other arm 55a is maintained in abutment at the bottom of the groove 26 a.

The control connecting rod is positioned in an intermediate position, asshown in FIG. 14, which positions its base 78 out of the reach of thestops 60 a and 61 a for pivoting the lever.

In order to control a change in the position of the segment 14 a, theservomotor guides the connecting rod so as to bring the base 78 alongthe trajectory of the extension stop 60 a so as to cause the pivoting ofthe lever 53 a with the rotation of the chainwheel. This position of thecontrol connecting rod is shown in FIG. 13.

In a first pivoting phase of the lever shown in FIG. 16, the head 58 aof the lever follows the slope 49 a of the ramp 48 a. The lever thenoperates a first shifting of the segment 14 a. The forward zone movesaway from the crankset axle until the pin 41 a reaches the end of theslot 40 a. The pin reaches the end of the slot 40 a at the same time asthe head of the lever reaches the connection of the two slopes 49 a and50 a. The position of the slope 49 a near the forward zone 34 a of thesegment and its sloping with respect to the lead angle of the head 58 aallow this first shifting movement of the forward zone 34 a.

In a second phase, the connecting rod continues to oppose the passage ofthe stop 60 a. The lever 53 a continues to pivot with the rotation ofthe chainwheel. Its head 58 a attacks the second slope 50 a of the ramp48 a and forces the rearward zone of the segment to be moved away fromthe crankset axle against the return force of the spring.

At the end of this second shifting phase, as shown in FIG. 17, the head58 a of the lever 53 a abuts against the return 51 a. The stop 60 aescapes from the connecting rod 76 through a flexion of the flexiblezone 79 of the base 78.

In this position of the lever 53, the arm 54 a prevents the segment frompivoting about an axis defined by the pin 41 a. The arm is orientedright beyond the line of action of the force exerted by the segment,such that the lever and the segment are maintained in a stable position.

The forward zone is also maintained in a stable position of balance.Indeed, in the absence of bias from the chain, the return force of thespring 66 a is sufficient for maintaining the lower end of thebean-shaped slot 40 a in support against the pin 41 a. When the segmentis subject to a tension from the chain, the resulting bias isessentially oriented along a tangent to the curve of the segment. Underthese conditions, the pin cooperates with the edge of its slot to regainthis tangent bias. The reaction that the pin exerts on the segmentreinforces the stable position of this forward zone in view of thesupport of the segment on the lever head, and the curve of thebean-shaped slot.

In this high position, the first tooth 37 a of the segment 14 a isspaced from the last tooth of the segment preceding it by one chainpitch.

If the connecting rod is maintained in its angular position of FIG. 12,it proceeds with the extension of the segments as the chainwheelrotates, and as the extension stops abut against its base.

Once all of the segments have been transferred to the extensionposition, the servomotor returns the connecting rod to its intermediateposition of FIG. 14, out of the reach of the indexing stops.

The transfer of the segments to their retraction position occurs bybringing the connecting rod to the position of FIG. 12 where it is onthe trajectory of the retraction stops, particularly the stop 61 a ofthe lever 53 a.

In their original position, all of the segments occupy a high positionshown in FIG. 18. By immobilizing the retraction stop, the connectingrod forces the lever 14 a to pivot so that its head 58 a leaves thesupport of the return 51 a and follows the ramp 50 a in the reversedirection. During this first phase, under the effect of traction of thespring, the segment shifts about the pin 41 a. Its rearward zone isbrought closer to the crankset axle until the notch 43 a takes supportagainst the guiding abutment 45 a.

The segment 14 a is in the same configuration as the one shown in FIG.16.

As the chainwheel follows its rotation, the lever 53 a is brought backto the position that it occupies in FIG. 15, thus releasing the forwardzone of the segment that is returned in the direction of the cranksetaxle by the residual traction of the spring.

In this position, the first tooth 37 a of the segment 14 a that isreturned to its low position is located at a distance of two chainpitches from the last tooth 39 d of the segment 14 d that precedes it.

The machining of the last space between the teeth previously mentionedeases the dipping of the chain.

Alternative constructions of the chainwheel are possible. In particular,the guiding of the forward zone of the segment could be obtained byother means. Specifically, one could invert the slot and the pin to havethe slot on the frame and the pin on the chainwheel. One could alsoinvert the movement of double shifting, i.e., raise the rearward zone ofthe segment first and the forward zone last. The movement of the segmentalong a tangent direction could have an inverted direction so that, onthe raising of the segment, there is a space of one chain pitch forward,rather than rearward, of the segment.

A single control lever is preferred for transferring the chainwheel fromone position to the other. As an alternative, one could have two levers,such as the lever 53 a, each responsible for controlling the raising orlowering of a sprocket zone. One could invert the functioning mode of alever so that its articulation is on the segment and the ramp is on theframe.

FIG. 19 schematically shows the electronic control system that guidesthe servomotor. This circuit has four magnets 85 a, 85 b, 85 c, 85 dthat are mounted on the chainwheel at a right angle one from the other.A Reed type relay 86 is affixed to the frame of the bicycle. The passageof a magnet in front of the relay corresponds to a specific position ofa control lever with respect to the base of the connecting rod.

The system has a control button 88 that can possibly be removed and aprocessing circuit 90. The processing circuit has a counting unit thatis capable of counting the locking impulses of the Reed relay, and aprocessing unit that guides the servomotor 82. Initially, the servomotoris in its intermediary position, and its last position for guiding theconnecting rod is saved.

Upon sending a command for changing the state of the chainwheel, as soonas the counting unit has counted an impulse, the processing unit guidesthe servomotor in the position opposite its last indexing position. Thenthe counting unit is actuated. As soon as it has counted four impulsescorresponding to a complete turn of the chainwheel, the processing unitguides the servomotor to return it to its intermediate position, andsaves the new guiding position from which the servomotor returns.

The servomotor is selected to be sufficiently quick so that a change ofposition of the connecting rod can occur before a new magnet is in frontof the Reed relay within a reasonable range of rotational speed of thechainwheel.

Other means for guiding the servomotor are also possible, just as othermeans for controlling the levers are also possible. In particular, onecould control the levers in a strictly mechanical way. One could alsodetect the passage of the segments other than by magnets and a Reedrelay.

Additionally, it is possible to couple the electronic control system toa system for controlling the rear derailleur, particularly when the rearderailleur has an electrical control for coordinating a gear shift atthe chainwheel with a shift at the rear derailleur, or even forcontrolling both transmission systems by means of a single control.

According to another means for using the control system, one couldcreate an intermediate state of the chainwheel in which one portion ofthe segments is in a high position and one portion is in the lowposition. In the case of four segments, one could have two diametricallyopposed segments in the high position, and the other two segments in ahigh position. Under these conditions, the chainwheel would functionsimilarly to an oval chainwheel.

FIG. 20 shows another constructional embodiment of the chainwheel. Thegeneral concept here is to displace each of the segments from oneposition to the other through a strictly rotational movement. The lowand high positions of a segment are identical to those that weredescribed previously. Since these positions are determined, it ispossible to identify an instantaneous center of rotation about which thesegment must pivot according to a strict rotation in order to pass fromone position to the other.

Thus, FIG. 20 shows four segments 94 a, 94 b, 94 c, 94 d. Each segment,particularly segment 94 a, is affixedly attached to the end of an arm 95a, that is itself connected by an articulation about an axle 96 a to theradial arm 97 a of a frame 98. The position of the axle 96 a wasdetermined such that this axle corresponds to the instantaneous centerof rotation of the segment. As can be seen in FIG. 20, the axle 96 a isapproximately at the intersection of a straight line tangent to theforward zone of the segment and of a straight line of radial directionpassing approximately through the middle of the segment locatedforwardly of said segment.

The change of position of each of the segments is controlled by a lever,such as the lever 99 a. This lever has a first arm 100 a that cooperateswith a slot 101 a of the segment to control the amplitude of the angularmovement about the axle 96 a, and two auxiliary arms that each bears anindexing stop 102 a, 103 a that allows controlling the shifting of thelever. This means for controlling the shifting of the lever is similarto what was described above, and the lever is provided to cooperate witha connecting rod that has not been described in detail.

In order to illustrate the invention, the segment 94 b is shown in thehigh position whereas the other segments are in the low position. Onecan see that at the junction between the segments 94 a and 94 b, theteeth are in correspondence with the chain pitch without an intermediarypitch. At the junction with the rearward segment 94 c, there is anintermediary chain pitch. Thus, the teeth of the chainwheel aremaintained in correspondence with the chain pitch on the raising as onthe lowering.

Another constructional method is schematically shown in FIGS. 21 and 22.According to this constructional embodiment, the trajectory of a segment105 a is guided by means of two cams 106 a, 107 a, one located in theforward zone, the other in the rearward zone. The cams pivot about axles108 a, 109 a mounted on the segment, and off-centered axles 110 a and111 a mounted on a crankset frame. Each cam has teeth 114 a, 115 a atits periphery.

The rotation of the cams can possibly be controlled in a synchronizedmanner by a pinion 119 a, also mounted on the chainwheel, and by meansof two stops 120 a, 121 a that operate according to the same functioningmode as the preceding stops. According to an alternative embodiment, thecams could be replaced by connecting rods.

The relative position of the axles 108 a, 110 a, 109 a, 111 a and theprimary diameter of the teeth 114 a and 115 a are determined such that,as in the preceding cases, the segment is displaced along a radialcomponent and a tangent component that maintain the teeth incorrespondence with the chain pitch during the transitory phases ofextension and retraction.

According to the alternative of FIGS. 23 and 24, the trajectory of asegment 120 a is guided by a shifter 121 a and a cam 122 a connectedtogether by a connecting rod 123 a. The shifter 121 a is pivotallymounted about an axle 125 a borne by the chainwheel frame. The frame isnot shown in FIGS. 23 and 24.

Toward the axle 125 a, the shifter has two arms 126 a, 127 a that areprovided to be fastened by an indexing member, for example, an indexingconnecting rod such as the connecting rod 76 described above, so as toguide the change of state of the segment.

The shifter 121 a furthermore has a head 128 a with a pin 129 a thatmoves in a slot 130 a cut out from the side of the rearward end of thesegment 120 a.

The slot 130 a generally has the form of an “S,” with a central portionoriented approximately in a radial manner, and two rounded ends for thetwo high and low positions of the segment.

Toward the other end, the segment is displaced by a cam 122 aarticulated about an axle 132 a borne by the frame and connected to theforward end of the segment 120 a by a pin 133 a. The pin 133 a isdisplaced by the cam on both sides of an imaginary circle centered onthe chainwheel axle and passing through the axle 132 a. The shifter 121a and the cam 122 a are connected by a connecting rod 123 a thatsynchronizes their movement.

The rotation of the shifter induces a displacement of the rearward endof the segment along radial and tangent components. The cam induces adisplacement of the segment mainly along a radial direction.

Optionally, the shifter 121 a and the cam 122 a are housed in housingscut into the chainwheel frame. The lateral walls of these housings serveas a force recovering abutment for the two extreme positions of theshifter and of the cam, so as to recover the forces induced by thetension of the chain.

As in the preceding cases in the low position, the first teeth of thevarious segments are juxtaposed to the last teeth of the segmentpreceding them so that the teeth of the various segments are continuousand in correspondence with the chain pitch. In the high position, theseteeth are spaced apart by one chain pitch. The segments are guidedindividually, and the change of position of each segment preferablyoccurs when the segment in question is in the raising zone of thesprocket.

The present description is only given as an example, and otherembodiments of the invention could be used without leaving the scopethereof.

In particular, the number of segments of the chainwheel is not limiting,and one could have more than or less than four segments. Furthermore,the number of teeth of the sprocket is not limiting either. An exampleof a chainwheel having a state of 44 teeth and an imaginary state of 48teeth has been described. The invention could be applied to a chainwheelof 48 teeth having an imaginary state of 52 teeth through an extensionof the segments, or yet a chainwheel of 49 teeth that can be extended to53 teeth by using non-identical segments, three having 12 teeth and one13 teeth.

Thus, the invention allows modifying the development of the fronttransmission system by changing the state of the chainwheel through anextension of its teeth. Since there is no lateral transfer of the chain,and since the variation of the development induced is relatively low,the change of state of the chainwheel occurs under very good conditionsof flexibility and smoothness, even when the chain is subject to a hightension.

Additionally, the energy necessary to the change of state of thechainwheel is primarily taken from the very rotation of the chainwheel.The servomotor, or another equivalent system, which controls a change ofstate of the chainwheel, consumes very low energy.

1. A drive sprocket for a bicycle crankset, said drive sprocketcomprising: a toothed ring, the ring comprising teeth having one pitchin correspondence with one chain pitch, the ring being divided into aplurality of segments, the segments being borne by a frame, the framebeing rotationally movable about a crankset axle, each said segmentbeing movable with respect to the frame in a plane perpendicular to thecrankset axle between a low position and a high position in which eachsaid segment is further distant from the crankset axle, thereby defininga small diameter state and a large diameter state of the sprocket, asystem for transferring each said segment from one of said low and highpositions to another of said low and high positions, and an indexingmechanism to control the system for transferring said segments, whereineach said segment is floatingly mounted with respect to the frame, andwherein said system for transferring each said segment between said lowand high positions transfers each said segment in guided movement alonga trajectory with respect to the frame, said trajectory of movementhaving a radial component and a tangent component, said tangentcomponent being greater than zero.
 2. A drive sprocket according toclaim 1, wherein: the plurality of segments consists of four segments of11 teeth each; in the low position, said four segments are adjacent eachother; in moving from the low position to the high position, eachadjacent pair of said four segments is moved apart by an amountcorresponding to one additional chain pitch.
 3. A drive sprocketaccording to claim 1, wherein: each of said segments has a forward zoneand a rearward zone, a forward zone of each of said segments following arearward zone of a preceding one of said segments; the radial componentof the trajectory of movement of each said segment, a in moving from thelow position to the high position, has a magnitude predetermined so thatin the high position all of said segments are spaced apart by a distancecorresponding to a whole number of a chain pitch; and the tangentcomponent of the trajectory of movement of each said segment, in movingfrom the low position to the high position, has a magnitudepredetermined so that a forwardmost tooth of each said segment issubstantially in correspondence, without an intermediary pitch, with arearwardmost tooth of an immediately preceding segment, while saidpreceding segment is in the low position.
 4. A drive sprocket accordingto claim 1, each segment having a forward zone and a rearward zone,wherein one of the zones of the segment is guided with respect to thegroove by a pin moving inside of a slot, and at least one shifting levercontrols movement of the other zone of the segment.
 5. A drive sprocketaccording to claim 4, wherein the movement of a segment is controlled bya single shifter having an arm supported against a ramp located at thebase of the segment.
 6. A drive sprocket according to claim 4, whereineach of the segments is elastically returned to the low position by areturn spring mounted between the segment and the frame.
 7. A drivesprocket according to claim 4, wherein the shifting lever has two arms,each bearing a stop for indexing the lever in one or the other of itspositions for controlling the displacement.
 8. A drive sprocketaccording to claim 7, wherein a connecting rod provided to be mounted onthe bicycle frame controls the shifting of the lever by positioningitself on the trajectory of one or the other of the indexing stops.
 9. Adrive sprocket according to claim 8, wherein the displacement of theconnecting rod is controlled by a member that is guided by an electroniccontrol system having a member for detecting each of the segments and aprocessing circuit having a counting unit and a processing unit to guidethe member for displacing the connecting rod.
 10. A drive sprocketaccording to claim 1, wherein each of the segments is affixed to the endof an arm that is itself connected to the radial arm of a frame about anaxle located at the instantaneous center of rotation.
 11. A drivesprocket according to claim 1, wherein the change of position of asegment is guided by two cams, a first of the two cams being located inthe forward zone, a second of the two cams being located in the rearwardzone of the sprocket, whose synchronized rotation is controlled by apinion.
 12. A drive sprocket according to claim 11, wherein saidsprocket has from 44 to 49 teeth in the small diameter state, and in thelarge diameter state, the sprocket has an effective expansion of 3 to 5additional teeth.
 13. A drive sprocket according to claim 1, wherein:said system for transferring each said segment between said low and highpositions comprises, for each said segment, the following: a shiftermounted to the frame for pivotal movement about an axis, said shifterbeing connected to one of a forward zone and a rearward zone of thesegment by a pin connected to a head of said shifter, and connected tothe other of the forward and rearward zones by a cam, said cam beingpivotally mounted to the frame and connected to the segment by a pin,the shifter and the cam being connected together by a connecting rod.14. A drive sprocket according to claim 1, wherein change from saidsmall diameter state to said large diameter state induces an expansionof approximately 10%.
 15. A transmission system for a bicycle having atransmission chain and a set of pinions with a rear derailleur totransfer the chain from one pinion to the other, wherein thetransmission system has a front drive sprocket according to claim
 1. 16.A transmission system according to claim 15, wherein the rear derailleuris an electrically controlled derailleur, wherein the rear derailleurand the drive sprocket are controlled by an electronic control systemcoordinating control of the rear derailleur and of the drive sprocket.17. A transmission system according to claim 15, wherein thetransmission system has a front sprocket that is extensible between twostates of 44 and 48 teeth, and a rear cassette of pinions having all orpart of a set of pinions of 11, 13, 15, 18, 21, 25, 30, 36 teeth.
 18. Adrive sprocket according to claim 1, wherein: said trajectory ofmovement of each said segment from said low to said high positionincludes a radial component of movement of a predeterminate extent awayfrom the crankset axle and a tangent component of movement of apredeterminate extent in a first direction transverse to said radialcomponent of movement; said trajectory of movement of each said segmentfrom said high position to said low position includes a radial componentof movement of a predeterminate extent toward the crankset axle and atangent component of movement of a predeterminate extent in a seconddirection opposite to said first direction.
 19. A drive sprocketaccording to claim 1, wherein: each of said segments comprises aplurality of teeth for meshing with and driving a chain.
 20. A drivesprocket according to claim 1, wherein: each of said segments isidentical to others of said segments.
 21. A drive sprocket for a bicyclecrankset, said drive sprocket comprising: a ring comprising a pluralityof teeth having a pitch corresponding with a pitch of a chain adapted tomesh with said drive sprocket for comprising a component of a bicycletransmission assembly; said toothed ring comprising plurality ofsegments, each of said plurality of segments having a plurality of saidplurality of teeth of said ring; a frame supporting said segments, saidframe adapted to be rotationally movable about a crankset axle of abicycle; a transfer system for movement of said plurality of segmentsrelative to said frame between a small diameter position of said drivesprocket and a large diameter position of said drive sprocket, each saidsprocket segment being spaced farther apart from an axis of rotation ofsaid drive sprocket in said large diameter position than in said smalldiameter position, said transfer system moving each said segmentsuccessively, one after another, during an extent of rotation of eachsaid segment about said axis of rotation corresponding to anon-engagement phase between said teeth of each said segment and saidchain; said transfer system comprising an arrangement of parts to guideeach of said sprocket segments successively along a predeterminate pathof movement, said predeterminate path of movement comprising acombination of radial movement and tangential movement greater thanzero, said radial movement being radial with respect to said axis ofrotation and said tangential movement being tangential with respect to acircle centered on said axis of rotation; an indexing mechanism tocontrol said transfer system for movement of said drive sprocket betweensaid small and large diameter positions.
 22. A drive sprocket accordingto claim 21, wherein: for each said segment, said transfer systemcomprises: a shifter mounted to said frame about a first pivot axis,said shifter being connected to one of a forward zone and a rearwardzone of the segment by means of a first pin spaced from said pivot axis,said shifter being connected to the other of the forward zone and therearward zone of the segment by means of a cam, said cam being mountedto said frame about a second pivot axis and connected to the segment bymeans of a second pin, the shifter and the cam being connected by meansof a connecting rod.
 23. A drive sprocket according to claim 21,wherein: in said large diameter position, said drive sprocket has aneffective diameter approximately 10% larger than in said small diameterposition.
 24. A drive sprocket for a bicycle crankset, said drivesprocket comprising: a ring comprising a plurality of teeth having apitch corresponding with a pitch of a chain adapted to mesh with saiddrive sprocket for comprising a component of a bicycle transmissionassembly; said toothed ring comprising plurality of segments, each ofsaid plurality of segments having a plurality of said plurality of teethof said ring; a frame supporting said segments, said frame adapted to berotationally movable about a crankset axle of a bicycle; a transfersystem for movement of said plurality of segments relative to said framebetween a small diameter position of said drive sprocket and a largediameter position of said drive sprocket, each said sprocket segmentbeing spaced farther apart from an axis of rotation of said drivesprocket in said large diameter position than in said small diameterposition, said transfer system moving each said segment successively,one after another, during an extent of rotation of each said segmentabout said axis of rotation corresponding to a non-engagement phasebetween said teeth of each said segment and said chain; said transfersystem comprising an arrangement of parts to guide each of said sprocketsegments successively along a predeterminate path of movement, saidpredeterminate path of movement comprising a combination of radialmovement and tangential movement, said radial movement being radial withrespect to said axis of rotation and said tangential movement beingtangential with respect to a circle centered on said axis of rotation;an indexing mechanism to control said transfer system for movement ofsaid drive sprocket between said small and large diameter positions;wherein: said plurality of segments consists of four segments, each ofsaid four segments having an identical number of teeth for meshing withthe chain, each of said segments following, in a direction of rotationabout said axis of rotation, a preceding one of said segments, each ofsaid segments having a forwardmost tooth and a rearwardmost tooth; insaid small diameter position of said drive sprocket, said segments areadjacent each other, whereby the forwardmost tooth of one of saidsegments is spaced behind the rearwardmost tooth of a preceding segmentby a distance corresponding to one pitch, or a whole number multiple ofsaid pitch, of the chain; in moving along said predeterminate path ofmovement from the small diameter position of said drive sprocket to thelarge diameter position of said drive sprocket, each of said segments ismoved substantially tangentially so that said forwardmost tooth of oneof said segments is spaced behind said rearwardmost tooth of saidpreceding segment by an additional distance corresponding to oneadditional pitch of said chain, or a whole number multiple of said pitchof said chain.
 25. A drive sprocket according to claim 24, wherein: eachof said four segments has 11 teeth, whereby, in said small diameterposition, said drive sprocket has 44 teeth, and in said large diameterposition, said drive sprocket has an effective 48 teeth, said effective48 teeth comprising said 44 teeth plus a distance of one pitch eachbetween successive ones of said four segments.